The long term goal of this project is an understanding of the structural and functionsl organization of the hypothalamus. Specific aims are 1) to determine whether the hypothalamus of the rhesus monkey is sexually differentiated in its cytoarchitectonic structure; and 2) to elucidate the development and functional correlations of the cytoarchitectonic sexual dimorphisms which we have found in the portion of the guinea pig hypothalamus involved in the regulation of sexually differentiated behavioral and neuroendocrinological reproductive functions. The study will discover whether or not the dimorphic patterns can be changed by prenatal, perinatal, and postnatal hormonal manipulations and whether such changes can be correlated with changes in sexually differentiated functions: patterns of luteinizing hormone (LH) release, ovulation, and mating behaviors. There will be a total of 19 treatment groups that receive various combinations of pre- and postnatal hormonal manipulations, including gonadectomy at birth or in adulthood, exposusre to androgen during one of several prenatal periods, and various postnatal hormonal treatments. The animals are given behavioral tests for mounting and lordosis following estrogen and progesterone priming and for mounting without priming. They are also examined for cyclic LH secretion by histological examination of ovaries, assessment by radioimmunoassay (RIA) of LH release following estrogen, and assessment of LH release by RIA following estrogen and progesterone. Following all functional tests, the brains of these animals are embedded in celloidin, sectioned and cell-stained. Finally, timemated female guinea pigs are given a single injection of tritiated thymidine on different days between days 20 and 32 of gestation in order to compare the time that neuronal birth and proliferation occurs in various brain regions with the periods during which androgen expossure influences specific sexually differentiated functions. Serial sections of the hypothalamus of every animial will be examined for changes in cytoarchitectonic patterns in the sexually dimorphic nu;clear complex of the medial preoptic and anterior hypothalamic areas (SDNC - MPAH). The quantitaive analysis of change in the volume of cell groups and in the size, number, or packing density of neurons within SDNC will be aided by computerized light microscopy.